As a positive working light sensitive layer to be solubilized by an active light radiation a light sensitive layer containing a photolytic acid generating compound and an acid decomposable compound is known. A light sensitive composition containing a water insoluble compound having a specific group capable of decomposed by an acid is disclosed in U.S. Pat. No. 3,779,779, a light sensitive composition containing a compound having an acetal or a ketal in the main chain is disclosed in Japanese Patent O.P.I. Publication No. 53-133429/1978, and a light sensitive composition containing a compound having a silylether group is disclosed in Japanese Patent O.P.I. Publication No. 65-37549/1985. These compounds have sensitivity in the ultraviolet range and are those capable of being solubilized by ultraviolet radiation, and can not give an image by irradiation of an infrared light such as a cheap and compact semiconductor laser.
As a technique for forming an image by exposure of infrared light such as a semiconductor laser, an image forming material comprising a light sensitive layer containing an acid generating compound, a resol resin, a novolak resin and an infrared absorber is disclosed in U.S. Pat. No. 5,340,699. The image forming material is imagewise exposed, heat-treated (pre-baked) before development, and then developed to form a negative image, or is imagewise exposed and then developed without the pre-baking to form a positive image. However, a planographic printing plate prepared by applying this technique is low in printing durability and therefore, additional baking (post-baking) after development is necessary, which is inconvenient to users. Further, the heat treatment after exposure and before development takes more processing time and lengthens the processing time. Further, the heat treatment requires high temperature, resulting in much electrical power consumption or more load to the processor.